1. Field of the Invention
The present invention relates to an image forming technique, and particularly to a technique that uses multi-pass printing to form an image with a lower tone than an input image.
2. Description of the Related Art
There are various types of recording apparatuses, which record desired information such as text, images, and so on onto a sheet-shaped recording medium such as recording paper or film, used as information output apparatuses in word processors, personal computers, facsimile devices, and so on. Among these, a type that forms text and images upon a recording medium by depositing printing material onto the recording medium has been put into practical use; a representative example of such a type is an image forming apparatus such as an inkjet recording apparatus.
In inkjet recording apparatuses, the heads are sometimes provided with nozzle groups, in which multiple ink nozzles capable of discharging the same color and density of ink are packed in an array in order to improve printing speed, enhance image quality, and so on. Furthermore, it is customary for such nozzle groups, which are capable of discharging inks of the same color but different densities, changing the discharge amount of inks of the same color and density between several levels, or the like, to be provided for each different color of ink. FIG. 17 is a diagram illustrating an example of printing using multiple nozzles. A multi-nozzle head 1701 includes eight nozzles 1702. The nozzles 1702 discharge ink droplets 1703 toward a print region 1704.
The error diffusion method proposed by R. Floyd et al. (“An adaptive algorithm for spatial gray scale,” SID International Symposium Digest of Technical Papers, Vol. 4.3, 1975, pp. 36-37) can be used by such an image forming apparatus as an image processing method that converts multi-valued input image data into print data, which corresponds to a recording signal for dots. This error diffusion method diffuses error arising in a pixel during a binary conversion process to peripheral pixels that will be processed thereafter, so that the data gradually changes from one color to another.
Furthermore, Japanese Patent Laid-Open No. 2002-096455 (U.S. Pat. No. 6,874,864) proposes a method regarding printing control for determining the formation order and arrangement when an image whose number of tones has been converted is formed by an image forming apparatus. With this technique, image processing is combined with the printing control, making it possible to form images at high speeds while suppressing degradation in image quality due to unprinted stripes, density unevenness, and so on. To be more specific, this patent document proposes a multi-pass recording method whereby multiple main scans in the same main scanning direction are made by different nozzle groups in the print regions of a predetermined recording medium and an image is formed as a result of the main scans.
Hereinafter, the multi-pass recording method shall be described with reference to FIG. 18. FIG. 18 is a diagram illustrating an example of the multi-pass recording method. With the multi-pass recording method, printing is performed by making multiple main recording scans in the print region 1704. Eight nozzles in the multi-nozzle head 1701 are divided into four nozzles on the upper side in the paper feed direction (an upper nozzle group), and four nozzles on the lower side in the paper feed direction (a lower nozzle group). The nozzle groups take turns making a single main recording scan in the same print region. Each nozzle 1702 prints approximately half of the number of dots contained in the image data, based on a predetermined image data arrangement for that image data. The printing of the image data is completed by filling in, during the second scan, the remaining dots of the image that was formed earlier.
A method that utilizes masking patterns exists as a method for calculating the print data for each pass. Each pass has a different masking pattern, and each pixel in the overall print region is allocated to one of the masking patterns. Only the pixels that correspond to the pixels allocated to the masking pattern of that pass are printed. A hound's tooth check pattern, illustrated in FIG. 19, is commonly used as the masking pattern. FIG. 19 is a diagram illustrating an example of a masking pattern. The printing of the print region is completed by a first scan that prints the hound's tooth check pattern and a second scan that prints the inverse of the hound's tooth check pattern. In the upper section of FIG. 19, the first pass of printing is performed in a print region 1901 using a lower nozzle group 1903. Here, only the dots corresponding to the hound's tooth pattern (the hatched circles) are printed. In the middle section of FIG. 19, the second pass of printing is performed in the print region 1901 using an upper nozzle group 1904. At the same time, the first pass of printing is performed in a print region 1902 using the lower nozzle group 1903. Here, only the dots corresponding to the inverse of the hound's tooth pattern (the white circles) are printed. Finally, in the lower section of FIG. 19, the second pass of printing is performed in the print region 1902 using the upper nozzle group 1904. Here, only the dots corresponding to the hound's tooth pattern (the hatched circles) are printed.
With such a multi-pass recording method, the ink discharge amount and the influence of unevenness in the discharge direction are halved, and thus overprinted stripes, unprinted stripes, and so on are not very noticeable in the formed image. This significantly alleviates density unevenness.
With the multi-pass recording method that uses masking patterns, the total of the print data of the passes is equivalent to the original binary image data. However, when a paper feed error occurs, the graininess of the image increases, and the quality of the output image dramatically drops. This phenomenon shall be described using FIG. 20. FIG. 20 is a diagram illustrating an example of the state of the multi-pass recording method in the case where a paper feed error has occurred. When the print data for each pass is calculated for a binary image 2001 using a pass masking pattern 2002, first pass print data 2003 and second pass print data 2004 are obtained. Here, in the case where a paper feed error has occurred during the printing of the second pass print data 2004, the print result ends up as the output image 2005. Compared to the original binary image 2001, the graininess of this image has increased significantly.
Degradation in the image quality with a multi-pass recording method that uses masking patterns causes the dispersity of the dots printed in each pass to decrease, and is thus also the cause of a drop in dispersity in the arrangement of dots in the output image when a paper feed error occurs.
The present invention increases the dispersity of the dot arrangement in an image formed using the multi-pass recording method, thereby reducing degradation in the image quality caused by a paper feed error occurring in the image forming apparatus.